The present invention relates to a D/A conversion method and apparatus suitable for DMT (Discrete Multi Tone) systems, for example ADSL (Asymmetric Digital Subscriber Line) systems and VDSL (Very high frequency Digital Subscriber Line) systems, or OFDM (Orthogonal Frequency Division Multiplex) systems.
A well known problem in D/A conversion is that often the digital samples that are to be D/A converted have a higher resolution (more bits) than the D/A converter used for the conversion.
One solution to this problem is to simply ignore the least significant bits of the digital samples and only use the bits that fit into the D/A converter. Thus, the samples are truncated before actual D/A conversion. This method, however, has the drawback that it raises the noise level of the resulting analog signal, since this truncation corresponds to a further quantization of the digital signal.
Another solution, described in U.S. Pat. No. 4,818,996, with reference to audio processing systems, tests whether samples have the most significant bits set to 0. If so, the samples are shifted before D/A conversion. In this way the least significant bits will be shifted into the conversion range of the D/A converter. After D/A conversion the resulting analog signal is attenuated to a corresponding degree to restore the correct signal level. However; as noted in U.S. Pat. No. 4,818,996, if this method is used on a sample for sample basis, it will introduce distortion due to the transients that are produced during switching of the attenuator between attenuating and non-attenuating states. In U.S. Pat. No. 4,818,996 this problem is somewhat reduced by requiring that a weak signal must exist for a certain period of time (predetermined number of samples) before subsequent samples are shifted and attenuated. This avoids too frequent switching back and forth between the two D/A conversion modes. However, the remaining mode switches will still produce distortion when they occur. This distortion will increase with the D/A conversion rate, since the mode switches will become more frequent at higher D/A conversion rates. Since the D/A conversion rate for DMT (for example ADSL or xDSL) systems is at least an order of magnitude higher than for audio signal processing, this method is not suitable in such applications.
An object of the present invention is to provide a D/A conversion method and apparatus suitable for DMT systems, such as xDSL systems, for example ADSL and VDSL systems, or OFDM systems that use the bit shift/attenuation or similar technique but avoid or reduce the distortion due to transients during mode switching to a minimum.
This object is achieved in accordance with the attached claims.
Briefly, an exemplary embodiment of the present invention uses the guard time period (cyclic prefix) between symbols to perform mode switches. If the sample having the largest magnitude of a frame needs to be shifted, the entire frame is shifted. Thus, mode switches will only be performed at frame boundaries. This restricts the transients to the guard time period, where they will not distort the useful signal.